The epidermal growth factor receptor (EGFR, Erb-B1) belongs to a family of proteins, involved in the proliferation of normal and malignant cells (Artega, C. L., J. Clin Oncol 19, 2001, 32-40). Overexpression of Epidermal Growth Factor Receptor (EGFR) is present in at least 70% of human cancers (Seymour, L. K., Curr Drug Targets 2, 2001, 117-133) such as, non-small cell lung carcinomas (NSCLC), breast cancers, gliomas, squamous cell carcinoma of the head and neck, and prostate cancer (Raymond et al., Drugs 60 Suppl 1, 2000, discussion 41-2; Salomon et al., Crit Rev Oncol Hematol 19, 1995, 183-232; Voldborg et al, Ann Oncol 8, 1997, 1197-1206). The EGFR-TK is therefore widely recognized as an attractive target for the design and development of compounds that can specifically bind and inhibit the tyrosine kinase activity and its signal transduction pathway in cancer cells, and thus can serve as either diagnostic or therapeutic agents. For example, the EGFR tyrosine kinase (EGFR-TK) reversible inhibitor, TARCEVA®, was recently approved by the FDA for treatment of NSCLC and advanced pancreatic cancer. Other anti-EGFR targeted molecules have also been approved such as IRESSA®.

Despite the early success of Tarceva, it has become clear that selectively targeting individual kinases can lead to the development of drug resistant tumors. Cells that have developed mutations within the drug/kinase binding pocket display a growth advantage in the presence of drug eventually leading to disease progression. Current clinical strategies aimed at combining these molecularly targeted drugs with standard chemotherapeutics, radiation, or other targeted agents will lead to novel strategies to improve overall response rate and increase the number of complete remissions.
Furthermore, elucidation of the complex and multifactorial nature of various diseases that involve multiple pathogenic pathways and numerous molecular components suggests that multi-targeted therapies may be advantageous over mono-therapies. Recent combination therapies with two or more agents for many such diseases in the areas of oncology, infectious disease, cardiovascular disease and other complex pathologies demonstrate that this combinatorial approach may provide advantages with respect to overcoming drug resistance, reduced toxicity and, in some circumstances, a synergistic therapeutic effect compared to the individual components.
Certain cancers have been effectively treated with such a combinatorial approach; however, treatment regimes using a cocktail of cytotoxic drugs often are limited by dose limiting toxicities and drug-drug interactions. More recent advances with molecularly targeted drugs have provided new approaches to combination treatment for cancer, allowing multiple targeted agents to be used simultaneously, or combining these new therapies with standard chemotherapeutics or radiation to improve outcome without reaching dose limiting toxicities. However, the ability to use such combinations currently is limited to drugs that show compatible pharmacologic and pharmacodynamic properties. In addition, the regulatory requirements to demonstrate safety and efficacy of combination therapies can be more costly and lengthy than corresponding single agent trials. Once approved, combination strategies may also be associated with increased costs to patients, as well as decreased patient compliance owing to the more intricate dosing paradigms required.
In the field of protein and polypeptide-based therapeutics it has become commonplace to prepare conjugates or fusion proteins that contain most or all of the amino acid sequences of two different proteins/polypeptides and that retain the individual binding activities of the separate proteins/polypeptides. This approach is made possible by independent folding of the component protein domains and the large size of the conjugates that permits the components to bind their cellular targets in an essentially independent manner. Such an approach is not, however, generally feasible in the case of small molecule therapeutics, where even minor structural modifications can lead to major changes in target binding and/or the pharmacokinetic/pharmacodynamic properties of the resulting molecule.
The use of EGFR inhibitors in combination with histone deacetylases (HDAC) has been shown to produce synergistic effects. Histone acetylation is a reversible modification, with deacetylation being catalyzed by a family of enzymes termed HDAC's. HDAC's are represented by X genes in humans and are divided into four distinct classes (J Mol Biol, 2004, 338:1, 17-31). In mammalians class I HDAC's (HDAC1-3, and HDAC8) are related to yeast RPD3 HDAC, class 2 (HDAC4-7, HDAC9 and HDAC10) related to yeast HDA1, class 4 (HDAC11), and class 3 (a distinct class encompassing the sirtuins which are related to yeast Sir2).
Csordas, Biochem. J., 1990, 286: 23-38 teaches that histones are subject to post-translational acetylation of the, ε-amino groups of N-terminal lysine residues, a reaction that is catalyzed by histone acetyl transferase (HAT1). Acetylation neutralizes the positive charge of the lysine side chain, and is thought to impact chromatin structure. Indeed, access of transcription factors to chromatin templates is enhanced by histone hyperacetylation, and enrichment in underacetylated histone H4 has been found in transcriptionally silent regions of the genome (Taunton et al., Science, 1996, 272:408-411). In the case of tumor suppressor genes, transcriptional silencing due to histone modification can lead to oncogenic transformation and cancer.
Several classes of HDAC inhibitors currently are being evaluated by clinical investigators. The first FDA approved HDAC inhibitor is Suberoylanilide hydroxamic acid (SAHA, Zolinza®) for the treatment of cutaneous T-cell lymphoma (CTCL). Other HDAC inhibitors include hydroxamic acid derivatives; PXD101 and LAQ824, are currently in the clinical development. In the benzamide class of HDAC inhibitors, MS-275, MGCD0103 and CI-994 have reached clinical trials. Moume et al. (Abstract #4725, AACR 2005), demonstrate that thiophenyl modification of benzamides significantly enhance HDAC inhibitory activity against HDAC1.
Recent advances suggest that EGFR-TK inhibitors in combination with HDAC inhibitors may provide advantageous results in the treatment of cancer. For example, co-treatment with SAHA significantly increased EGFR2 antibody trastuzumab-induced apoptosis of BT-474 and SKBR-3 cells and induced synergistic cytotoxic effects against the breast cancer cells (Bali, Clin. Cancer Res., 2005, 11, 3392). HDAC inhibitors, such as SAHA, have demonstrated synergistic antiproliferative and apoptotic effects when used in combination with gefitinib in head and neck cancer cell lines, including lines that are resistant to gefitinib monotherapy (Bruzzese et al., Proc. AACR, 2004). Pretreating gefitinib resistant cell lines with the HDAC inhibitor, MS-275, led to a growth-inhibitory and apoptotic effect of gefitinib similar to that seen in gefitinib-sensitive NSCLC cell lines including those harboring EGFR mutations (Witta S. E., et al., Cancer Res 66:2, 2006, 944-50). The HDAC inhibitor PXD101 has been shown to act synergistically to inhibit proliferation with the EGFR1 inhibitor Tarceva® (erlotinib) (WO2006082428A2).
Current therapeutic regimens of the types described above attempt to address the problem of drug resistance by the administration of multiple agents. However, the combined toxicity of multiple agents due to off-target side effects as well as drug-drug interactions often limits the effectiveness of this approach. Moreover, it often is difficult to combine compounds having differing pharmacokinetics into a single dosage form, and the consequent requirement of taking multiple medications at different time intervals leads to problems with patient compliance that can undermine the efficacy of the drug combinations. In addition, the health care costs of combination therapies may be greater than the cost of single molecule therapies. Furthermore, it may be more difficult to obtain regulatory approval of a combination therapy since the burden for demonstrating activity/safety of a combination of two agents may be greater than for a single agent (Dancey J & Chen H, Nat. Rev. Drug Dis., 2006, 5:649). The development of novel agents that target multiple therapeutic targets selected not by virtue of cross reactivity, but through rational design will help improve patient outcome while avoiding these limitations. Thus, enormous efforts are still directed to the development of selective anti-cancer drugs as well as to new and more efficacious combinations of known anti-cancer drugs.